[llvm] Allow .alt_entry symbols to pass the .cfi nesting check (PR #82268)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

> Fixes: https://github.com/llvm/llvm-project/issues/82261

Improve the description by saying that a symbol with the N_ALT_ENTRY attribute can be defined in the middle of a subsection. Therefore it is reasonable to opt out the `.cfi_startproc` nesting check?

https://github.com/llvm/llvm-project/pull/82268